just pick up an escort gt-1 for $40 from amazon. it lets you know the 0-X speed in intervals of 10, and it also tells you what acceleration numbers you are pulling in each gear. very easy to check if a tune has made even the most minor difference with that feature, because its not launch dependent. very consistent.

Does it matter? if the lag is constant; 0-100 will still be the same. I mean if there is a lag of detecting the starting point, if the same lag happens at 100kmh, that is ok. So it is not the major problem here. From experience looking at the GPS speed itself on screen. It is very quick and almost instant.

The problem is really getting the GPS realising the car is starting off from standing start. Most GPS unit has static navigation function so when they plotting graph in traffic and do not plot too many dots around a static point (when the object/car is motionless). That is why never use the internal GPS on the N95 for the 0-100kmh, rather do 80-120kmh or something similar.

That is why Antii from RaceChrono incooperate the OBD function. The OBDkey can get 4Hz in most car (and in my RX-8 I manage to get 8Hz), so they update data a lot of times. It will be more accurate.
EDIT: OBD = on board diagnostic unit. It will get the speedo from the ECU itself.

The unit is best when use in lap times; it is proven even in the 1Hz mode that the lap time is within 0.1 second of the AIMS infrared transponder or the real magnetic transponder on the racetrack (natsoft ones).